Advances in plasma processing have facilitated growth in the semiconductor industry. In the competitive semiconductor industry, a manufacturer may gain a competitive edge if the manufacturer has the ability to maximize throughput and/or to produce quality devices. One method for controlling throughput is to control the flow of gas into the processing chamber.
Typically, for substrate processing, a recipe may require more than one gas species. Ideally, the gas species become mixed and reach an equilibrium pressure state (e.g., set pressure) within a processing chamber at the same time. However, several factors may cause the gas species to have different time scales (i.e., delivery time).
One factor that may impact the gas delivery time is the mass of gas species. Those skilled in the art are aware that gas species with heavier molecular mass may travel slower than gas species with lighter molecular mass. The mass difference between gas species may impact the flow rate of each gas specie in a low pressure environment. In a low pressure environment, the gas flow may become molecular and each gas specie may become virtually independent of each other. As a result, separation of the gas species may occur resulting in gas composition drift at the chamber. In other words, the gas species may reach the equilibrium state at different time. Thus, the time scale (e.g., delivery time) for each gas may differ.
Another factor that may impact the gas delivery time scale is the gas line geometry. As aforementioned, a recipe may require more than one gas species to perform substrate processing. Each gas may flow from a gas line into a mixing manifold (main gas line). The geometry of each gas line may impact the flow of the gas. As an example, the delivery time will be greater for the gas flowing through the longer gas line.
Some recipes may have low-flow gas mixing with high-flow gas. This type of gas delivery is known as a carrier gas-driven delivery, where the high-flow gas (carrier gas) drives the flow of the low-flow gas (process gas) via molecular collision. In order for the process gas to enter the mixing manifold where the carrier gas is flowing, the process gas needs to build up a pressure comparable to the pressure at the mixing manifold. However, if the carrier gas is flowing at a much higher flow rate than the process gas, it could take a prohibitively long time for the process gas to build up enough pressure and then mix with the carrier gas. In this case, the carrier gas will reach the processing chamber without carrying the process gas. Thus, the carrier will reach an equilibrium state before the process gas resulting in gas composition drift.
As can be appreciated from the foregoing, undesirable consequences may result due to the gas composition drift. For most recipes, substrate processing may begin when pressure stabilization has been reached within the processing chamber, regardless of gas composition drift. Performing substrate processing without the proper gas mixture may cause substandard devices to be created. Other recipes may require each gas specie within the processing chamber to reach the required equilibrium state before processing may begin. However, the additional time required may result in longer processing time and less substrate to be processed.